The present invention relates to an optical wave guide device usable for optical communications and a method of forming the same.
As a great deal of rapid and worldwide propagation of internet, the requirement for commercialization of the optical communication devices has been on the increase. For example, 2.5 Gb/s-systems capable of transmissions with large capacity corresponding to thirty thousands telephone lines have been introduced into many areas As the amount of informations to be transmitted through the optical communication system has been on the increase, a wavelength-multiplexing system has been practiced. In the initial state, a few waves were multiplexed in wavelength. Recently, however, a high density wavelength-multiplexing system could be realized at about 80 wave level. For the high density wavelength-multiplexing system, there are important a multiplexer for multiplexing plural optical signals having different wavelengths to introduce the multiplexed optical signal into a single optical fiber or a single optical waveguide as well as a demultiplexer for demultiplexing the multiplexed optical signal into plural optical signals having different wavelengths to introduce the plural optical signals into plural optical fibers or plural optical wave guides. An array wave-guide grid has been attracted as one example. FIG. 1 is a diagram illustrative of a conventional array wave-guide grid. The conventional array wave-guide grid has an input waveguide 21, an array waveguide 23 connected through a first star-coupler 23 to the input waveguide 21 and an, output waveguide 25 connected through a second star-coupler 24 to the array waveguide 23. The array waveguide 23 comprises plural wave guides which are different in length of optical path by a constant difference and are arranged in array, so that the array waveguide 23 serves as a high-order diffraction-grating to exhibit multiplexing and demultiplexing functions. The array waveguide grid has already been commercialized and used in the existent optical communication system, wherein silica-based optical wave guides are formed over a silicon substrate or a silica substrate.
If the silica-based optical wave guides are formed over the silicon substrate, then a difference in thermal expansion coefficient between the silica-based optical wave guides and the silicon substrate causes a thermal stress which generates a birefringence or birefringence or double refraction in a silica-based layer of the silica-based optical wave guide. The birefringence or birefringence or double refraction causes a difference in propagation characteristics of the silica-based optical wave guide between in TE-mode and TM-mode. Particularly, this problem is serious to the device such as the array waveguide grid having a narrow distance of adjacent channel wavelengths and an abrupt transmission wavelength spectrum because a slight difference in wavelength characteristics between in TE-mode and TM-mode causes a large polarization dependency loss. In case of the array waveguide grid device with a frequency distance of 100 GHz, the polarization dependency loss is approximately proportional to a difference xcex94xcex(=xcexTMxe2x88x92xcexTE) between a first transmission center wavelength xcexTM in TM-mode and a second transmission center wavelength xcexTE in TE-mode. A proportional constant may be estimated in the range of about a few dB/nm to 10 dB/nm unless any specific design technique is taken to planarize a peak portion of the transmission wavelength spectrum. If the silica-based optical waveguide, which have practically been manufactured, is applied to the above described array waveguide grid device, then the polarization dependency loss is extremely large, for example, not less than 1 dB since xcex94xcex is, normally not less than 0.1 nm. The actually used array wave guide grid device having the silicon substrate and the silica-based optical wave guide is further provided with a half-wavelength plate at a center of the array wave guide for canceling a difference in wavelength characteristic between polarized lights. FIG. 2 is a diagram illustrative of the conventional array wave guide grid device. The conventional array wave guide grid devices has an input waveguide 21, an array waveguide 23 connected through a first star-coupler 23 to the input waveguide 21 and an output waveguide 25 connected through a second star-coupler 24 to the array waveguide 23. The array waveguide 23 comprises plural wave guides which are different in length of optical path by a constant difference and are arranged in array, so that the array waveguide 23 serves as a high-order diffraction-grating to exhibit multiplexing and demultiplexing functions. The array waveguide 23 also has a half-wavelength plate 26 at its center position for canceling a difference in wavelength characteristic between polarized lights. The additional provision of the half-wavelength plate 26 suppresses the polarization dependency loss to not more than 0.2 dB which is not practical problem. It is, however, necessary to realize a highly accurate positioning of the half-wavelength plate 26 through many additional processes. The half-wavelength plate is somewhat expensive and makes it difficult to reduce the manufacturing cost of the conventional array wave guide grid device. In this circumstances, it had been required to reduce the polarization dependency loss without the half-wavelength plate. In order to reduce the polarization dependency loss without the half-wavelength plate, it is necessary to reduce the thermal stress to the silica-based layer of the optical wave guide. In order to reduce the thermal stress, it is effective that a dopant concentration of a dopant such as phosphorus or boron of the silica-based layer is adjusted so that the thermal expansion coefficient of the doped silica-based layer is made close to the thermal expansion coefficient of tie silicon substrate. In Japanese laid-open patent publication No. 8-136754, it is disclosed that in order to reduce the thermal stress, a cladding, layer is used to form the optical wave guide, wherein a dopant concentration of a dopant such as phosphorus or boron of the silica-based cladding layer is adjusted so that the thermal expansion coefficient of the silica-based cladding layer is made close to the thermal expansion coefficient of the Silicon substrate. FIG. 3 is a diagram illustrative of individual variations in thermal expansion coefficients of silica-based glasses doped with individual dopants, for example P2O5, GeO2, B2O3, Al2O3, F and TiO2, over a dopant concentration. It is possible to reduce the difference in thermal expansion coefficient between the silica-based glass layer and the silicon substrate by controlling the dopant concentration.
In ELECTRONICS LETTERS vol. 33, No. 13, pp. 1173-1174, June 1997, it is disclosed that the array wave guide grid device has a reduced stress birefringence or double refraction, wherein the transmission center wavelength difference xcex94xcex is reduced from 0.19 nanometers to 0.03 nanometers.
In accordance with the conventional method, the silica-based film is formed by use of a high temperature heat treatment at a temperature of not less than 1200xc2x0 C., for example, FHD method, wherein in order to reduce the stress, dopant concentrations of P and B are somewhat increased from the normal concentrations. The high temperature heat treatment causes separated phases of P2O5 and B2O3 in the silica-based glass layer. A large amount of deposition is formed in the layer or on a surface of the layer. The separated phases and the deposition serve for scattering the light, whereby the optical propagation loss is increased.
In the above circumstances, it had been required to develop an optical waveguide device with a reduced polarization dependency and an optical propagation loss as well as a method of forming the device by use of low temperature processes with an optimization to the dopant concentration in the layer of the stress in the layer
The above conventional techniques further have the following disadvantages. In order to reduce the stress of the wave guide layer, the bottom cladding layer underlying the waveguide layer comprises the silica-based layer doped with P and B at high concentrations, for which reason if the high temperature heat treatment is carried out after the top cladding layer has been formed, then a core in the wave guide is dropped into the bottom cladding layer. Namely, it has been known that if the silica-based layer is heavily doped with the dopants such as P and B at high concentrations, then a softening temperature of the silica-based layer is dropped During the high temperature heat treatment cared out after the top cladding layer has been formed, the silica-based layer is made unstable so that the core of the wave guide is displaced or tilted, whereby the device performances are deteriorated. Particularly, in case of the array wave guide grid device, a slight displacement or a slight tilting of the core allows an increased cross-talk between adjacent channels. In Japanese laid-open patent publication. No. 5-157925, it is disclosed that in order to prevent the displacement or tilting of the core, a natural silica glass layer or a pure-silica glass layer free of any dopiest is formed on a top surface of the bottom cladding layer, wherein the top cladding layer is formed at a high temperature of not less than 1000xc2x0 C., for which reason the natural silica glass layer is needed to have a sufficient thickness. The natural silica glass layer has a high softening temperature and a high rigidity but has a large stress for bending the substrate or for increasing the stress in the wave guide layer, resulting in the increase in the transmission center wavelength difference xcex94xcex. Further, it is difficult to control the reflective index of the natural silica glass layer, for which reason the other reflective index of the cladding layer is adjusted to the reflective index of the natural silica glass layer by controlling the dopant concentration of the cladding layer and selecting the kind of the dopant. As a result, the freedoms of the kind of the dopant and he dopant concentration of the cladding layer are limited.
In the above circumstances, it had been required to develop a novel optical waveguide device with a reduced polarization dependency and high device performances and superior substrate in-plane uniformity as well as a method of forming the same.
Consequently, the above circumstances, it had been required to develop a novel optical waveguide device and a method of forming the same free from the above problems.
Accordingly, it is an object of the present invention to provide a novel optical waveguide device free from the above problems.
It is a further object of the present invention to provide a novel optical waveguide device with a reduced polarization dependency and high device performances and superior substrate in-plane uniformity.
It is a still further object of the present invention to provide a novel optical waveguide device with a reduced polarization dependency and an optical propagation loss as well as a method of forming the device by use of low temperature processes with an optimization to the dopant concentration in the layer of the stress in the layer.
It is yet a further object of the present invention to provide a novel method of forming an optical waveguide device free from the above problems.
It is further more object of the present invention to provide a novel method of forming an optical waveguide device with a reduced polarization dependency and high device performances and superior substrate in-plane uniformity.
It is more over object of the present invention to provide a novel method of forming an optical waveguide device with a reduced polarization dependency and an optical propagation loss as well as a method of forming the device by use of low temperature processes with an optimization to the dopant concentration in the layer of the stress in the layer.
The first present invention provides an optical waveguide device having a bottom cladding layer, a core and a top cladding layer, wherein a first submerge-preventing silica-based layer is further provided over the bottom cladding layer and under the core, and the first submerge-preventing silica-based layer is doped with at least one dopant and the first submerge-preventing silica-based layer is higher in softening temperature than the top cladding layer.
The second present invention provides a method of forming an optical waveguide device, comprising the steps of forming a bottom cladding layer over a silicon substrate; forming a first submerge-preventing silica-based layer over the bottom cladding layer; selectively forming a core on the first submerge-preventing silica-based layer; forming a top cladding layer, which comprises a first silica-based layer doped with at least one dopant selected from the group consisting of phosphorus and boron, on the core and over the first submerge-preventing silica-based layer by a chemical vapor deposition method, wherein the first silica-based layer is lower in softening temperature than the first submerge-preventing silica-based layer; and subjecting the first silica-based layer to a heat treatment in the range of 800-1000xc2x0 C.
The third present invention provides an optical waveguide device having a bottom cladding layer, a core and a top cladding layer, wherein at least the top cladding layer comprises a first silica-based layer doped with at least one dopant so that the first silica-based layer has a total dopant concentration in the range of 8.8 percents by weight to 15 percents by weight, and the first silica-based layer has a difference of not more than 0.03 nanometers in transmission center wavelength which depends upon polarization of the optical waveguide device.
The fourth present invention provides a method of forming an optical waveguide device comprising the steps of: forming a bottom cladding layer over a silicon substrate, selectively forming a core on the bottom cladding layer; forming a top cladding layer, which comprises a first silica-based layer doped with at least one dopant selected from the group consisting of phosphorus and boron, on the core and over the bottom cladding layer by a chemical vapor deposition method; and subjecting the first silica-based layer to a beat treatment in the range of 800-1000xc2x0 C.
The above and other objects, features and advantages of the present invention will be apparent from the following descriptions.